1. Field of the Invention
The invention relates to fungal compositions and methods of using them for control of subterranean termites.
2. Description of the Prior Art
Subterranean termites are particularly destructive pests in tropical and temperate regions throughout the world. In the United States alone, subterranean termites are estimated to cause $1 billion in damage annually including prevention and repair costs. They are known to infest cellulose-based materials including living trees, wooden structures, plant roots and books. One predominant species, the Formosan subterranean termite (FST), Coptotermes formosanus (Shiraki), has become an economically significant pest in the United States in the past 50 years. Reasons for this include their massive colonies which can contain tens of millions of individuals, their ability to attack several species of living trees, and their high level of reproduction. The Formosan subterranean termite is thought to have been transported to the US mainland at the end of World War II when military equipment was shipped back in wooden crates. The infestations have since radiated from the port cities of New Orleans and Lake Charles, La., Houston, Tex. and Charleston, S.C. The cryptic nature of the insects allowed them to establish colonies without being detected and made it difficult to determine the most effective treatment location. The extent of FST infestations has become apparent in dense swarms of flying termites and significant damage to buildings and trees.
Organochlorine compounds were previously used to control FST, but their sale was banned in 1988. Replacement chemicals are not as persistent [Su et al., Pest Managem. Rev. (1998) 3: 1-13]. In addition, by disturbing soil around a structure when landscaping or compensating for soil subsidence the chemical barriers can be compromised and allow FST access to the structure [Su et al., (1990) Sociobiology 17: 77-94]. Su et al. (1998, supra) review some alternative control methods including non-repellant termiticides and bait technology. In order for these techniques to work they must not repel termites, must be easily transferrable in or on termite bodies and have delayed toxicity which allows transfer from foraging workers to members of the termite colony that do not forage [Sociobiology (1996) 27: 253-275 and 1998, supra].
One alternative to chemical control entails use of biological control agents [Culliney et al., Bulletin of Entomological Research (2000) 90: 9-211]. Bacteria, viruses, protozoa and fungi have potential as pathogenic agents. Fungi exhibit qualities which can make them ideal for this application, including a slow-acting nature similar to that of successful chemicals, the ability to self-replicate and the ability of fungal spores to be spread by termite social behavior [Grace et al. (1992) Sociobiology 20: 23-28]. Milner et al. [Biocontrol Science and Technology (1966) 6: 3-9] review a wide variety of fungal pathogens that have been reported as potential pathogens to termites. Pathogenicity of strains of both Metarhizium anisopliae (Metschnikoff) Sorokin and Beauveria bassiana (Balsamo) Vuillemin have been demonstrated in laboratory colonies of C. formosanus [Delate et al. (1995) J. Appl. Entomol. 119:, 429-433; Wells et al. (1995) J. Entomol. Sci. 30: 208-215]. Jones et al. [Environ. Entomol. (1996) 25:, 481-487] discovered that small numbers of B. bassiana and M. anisopliae spores can be spread throughout a C. formosanus colony without being detected by the termites. Conditions in a termite nest, moderate temperature and high humidity, are conducive to the growth of fungal species and are important factors in fungal survivability and propagation [Kramm et al. (1982) J. Invertebr Pathol 39: 1-5.; Ignoffo (1992) Florida Entomol. 75: 516-525]. Stimac et al. (U.S. Pat. No. 6,280,723) teach a novel B. bassiana strain (AATCC 20872) useful in controlling termites of the genera Cryptotermes, Coptotermes, Incistermes, and Reticulitermes. Grooming and other social activity between termites facilitate the spread of fungal infection throughout a colony, which may result in elimination of a colony or a drastic reduction in its numbers and potential to cause economic damage. However, defensive actions such as avoidance of fungi, the removal and burial of fungus-killed termite cadavers and various immune responses can limit the spread of infection in the colony.
Baits containing effective entomopathogenic agents may allow the “horizontal transmission” of a fungal pathogen from termite to termite and eventual spread to the entire colony. They would provide long-term control or suppression of termite infestations. The fungal isolate, dose, termite species and individual termite colony may all be factors that determine if there is repellency due to the presence of the fungus, and the degree of repellency. If spores are repellent, there will be less horizontal transmission. Bait formulation additives may be required to overcome the repellency.
It may be preferable that an entomopathogenic fungus intended for use as a biocontrol agent for termites have an effective, but relatively slow, mode of action. This will allow the fungus to become more widely dispersed throughout the colony before mortality occurs. A highly virulent fungus may only kill the termites in the immediate vicinity of the bait.